Typhoon

Three different tropical cyclones spinning over the western Pacific Ocean on August 7, 2006. The cyclone on the lower right has intensified into a typhoon.

A typhoon is a mature tropical cyclone that develops in the western part of the North Pacific Ocean between 180° and 100°E. This region is referred to as the Northwestern Pacific Basin,[1] and is the most active tropical cyclone basin on Earth, accounting for almost one-third of the world's annual tropical cyclones. For organisational purposes, the northern Pacific Ocean is divided into three regions: the eastern (North America to 140°W), central (140° to 180°W), and western (180° to 100°E). The Regional Specialized Meteorological Center (RSMC) for tropical cyclone forecasts is in Japan, with other tropical cyclone warning centers for the northwest Pacific in Honolulu (the Joint Typhoon Warning Center), the Philippines and Hong Kong. While the RSMC names each system, the main name list itself is coordinated among 18 countries that have territories threatened by typhoons each year. The Philippines use their own naming list for systems approaching the country.

Within the northwestern Pacific there are no official typhoon seasons as tropical cyclones form throughout the year. Like any tropical cyclone, there are six main requirements for typhoon formation and development: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure center, a pre-existing low level focus or disturbance, and low vertical wind shear. The majority of storms form between June and November while tropical cyclone formation is at a minimum between December and May. On average, the northwestern Pacific features the most numerous and intense tropical cyclones globally. Like other basins, they are steered by the subtropical ridge towards the west or northwest, with some systems recurving near and east of Japan. The Philippines receive the brunt of the landfalls, with China and Japan being impacted slightly less. Some of the deadliest typhoons in history have struck China. Southern China has the longest record of typhoon impacts for the region, with a thousand-year sample via documents within their archives. Taiwan has received the wettest known typhoon on record for the northwest Pacific tropical cyclone basin.

The term Typhoon is the regional name in the northwest Pacific for a severe (or mature) tropical cyclone,[2] whereas hurricane is the regional term in the northeast Pacific and northern Atlantic.[3] Elsewhere this is called a tropical cyclone, severe tropical cyclone, or severe cyclonic storm.[4]

A tropical depression is the lowest category that the Japan Meteorological Agency uses and is the term used for a tropical system that has wind speeds not exceeding 33 knots (38 mph; 61 km/h).[7] A tropical depression is upgraded to a tropical storm should its sustained wind speeds exceed 34 knots (39 mph; 63 km/h). Tropical storms also receive official names from RSMC Tokyo.[7] Should the storm intensify further and reach sustained wind speeds of 48 knots (55 mph; 89 km/h) then it will be classified as a severe tropical storm.[7] Once the system's maximum sustained winds reach wind speeds of 64 knots (74 mph; 119 km/h), the JMA will designate the tropical cyclone as a typhoon—the highest category on its scale.[7]

From 2009 the Hong Kong Observatory started to further divide typhoons into three different classifications: typhoon, severe typhoon and super typhoon.[8] A typhoon has wind speed of 64-79 knots (73-91 mph; 118–149 km/h), a severe typhoon has winds of at least 80 knots (92 mph; 150 km/h), and a super typhoon has winds of at least 100 knots (120 mph; 190 km/h).[8] The United States' Joint Typhoon Warning Center (JTWC) unofficially classifies typhoons with wind speeds of at least 130 knots (67 m/s; 150 mph; 241 km/h)—the equivalent of a strong Category 4 storm in the Saffir-Simpson scale—as super typhoons.[9] However, the maximum sustained wind speed measurements that the JTWC uses are based on a 1-minute averaging period, akin to the U.S.' National Hurricane Center and Central Pacific Hurricane Center. As a result, the JTWC's wind reports are higher than JMA's measurements, as the latter are based on a 10-minute averaging interval.[10]

There are six main requirements for tropical cyclogenesis: sufficiently warm sea surface temperatures, atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low pressure center, a pre-existing low level focus or disturbance, and low vertical wind shear. While these conditions are necessary for tropical cyclone formation, they do not guarantee that a tropical cyclone will form. Normally, an ocean temperature of 26.5 °C (79.7 °F) spanning through a depth of at least 50 metres (160 ft) is considered the minimum to maintain the special mesocyclone that is the tropical cyclone.[citation needed] These warm waters are needed to maintain the warm core that fuels tropical systems. A minimum distance of 500 km (300 mi) from the equator is normally needed for tropical cyclogenesis.[11]

Whether it be a depression in the intertropical covergence zone (ITCZ) or monsoon trough, a broad surface front, or an outflow boundary, a low level feature with sufficient vorticity and convergence is required to begin tropical cyclogenesis. About 85 to 90 percent of Pacific typhoons form within the monsoon trough.[12] Even with perfect upper level conditions and the required atmospheric instability, the lack of a surface focus will prevent the development of organized convection and a surface low. Vertical wind shear of less than 10 m/s (20 kn, 33 ft/s) between the ocean surface and the tropopause is required for tropical cyclone development.[11] Typically with Pacific typhoons, there are two outflowjets: one to the north ahead of an upper trough in the Westerlies, and a second towards the equator.[12]

In general, westerly wind increases associated with the Madden-Julian Oscillation lead to increased tropical cyclogenesis in all tropical cyclone basins. As the oscillation propagates from west to east, it leads to an eastward march in tropical cyclogenesis with time during that hemisphere's summer season.[13] On average, twice per year twin tropical cyclones will form in the western Pacific ocean, near the 5th parallel north and the 5th parallel south, along the same meridian, or line of longitude.[14] There is an inverse relationship between tropical cyclone activity in the western Pacific basin and the north Atlantic basin, however. When one basin is active, the other is normally quiet, and vice versa. The main reason for this appears to be the phase of the Madden-Julian oscillation, or MJO, which is normally in opposite modes between the two basins at any given time.[15]

Nearly one-third of the world's tropical cyclones form within the western Pacific. This makes this basin the most active on Earth.[17] Pacific typhoons have formed year round, with peak months from August to October. The peak months correspond to that of the Atlantic hurricane seasons. Along with a high storm frequency, this basin also features the most globally intense storms on record. One of the most recent busy seasons was 2013. Tropical cyclones form in any month of the year across the northwest Pacific ocean, and concentrate around June and November in the northern Indian ocean. The area just northeast of the Philippines is the most active place on Earth for tropical cyclones to exist. Across the Philippines themselves, activity reaches a minimum in February, before increasing steadily through June, and spiking from July through October, with September being the most active month for tropical cyclones across the archipelago. Activity falls off significantly in November, although Typhoon Haiyan, the strongest Philippine typhoon on record, was a November typhoon.[18] The most frequently impacted areas of the Philippines by tropical cyclones are northern and central Luzon and eastern Visayas.[19] A ten-year average of satellite determined precipitation showed that at least 30 percent of the annual rainfall in the northern Philippines could be traced to tropical cyclones, while the southern islands receive less than 10 percent of their annual rainfall from tropical cyclones.[20]

Most tropical cyclones form on the side of the subtropical ridge closer to the equator, then move poleward past the ridge axis before recurving north and northeast into the main belt of the Westerlies.[21] When the subtropical ridge position shifts due to El Niño, so will the preferred tropical cyclone tracks. Areas west of Japan and Korea tend to experience much fewer September–November tropical cyclone impacts during El Niño and neutral years. During El Niño years, the break in the subtropical ridge tends to lie near 130°E, which would favor the Japanese archipelago.[22] During La Niña years, the formation of tropical cyclones, along with the subtropical ridge position, shifts westward across the western Pacific ocean, which increases the landfall threat to China and greater intensity to Philippines.[22] Those that form near the Marshall Islands find their way to Jeju Island, Korea.[23]

The list of names consists of entries from 17 Southeast and East Asian nations and the United States who have territories directly affected by typhoons. The submitted names are arranged into five lists; and each list is cycled with each year. Unlike tropical cyclones in other parts of the world, typhoons are not named after people. Instead, they generally refer to animals, flowers, astrological signs, and a few personal names. However, PAGASA retains its own naming list, which does consist of human names.[27] Therefore, a typhoon can possibly have two names. Storms that cross the date line from the central Pacific retain their original name, but the designation of hurricane becomes typhoon. In Japan, typhoons are also given a numerical designation according to the sequence of their occurrence in the calendar year.[24]

The most active Western Pacific typhoon season was in 1964,[citation needed] when 39 storms of tropical storm strength formed. The least activity seen in the northwest Pacific ocean was during the 2010 Pacific typhoon season, when only 14 tropical storms and seven typhoons formed. In the Philippines, the most active season, since 1945, for tropical cyclone strikes was 1993 when nineteen tropical cyclones moved through the country.[28] There was only one tropical cyclone that moved through the Philippines in 1958.[29] The 2004 Pacific typhoon season was the busiest for Okinawa since 1957.[30] Within Guangdong in southern China, during the past thousand years, the most active decades for typhoon strikes were the 1660s and 1670s.[31]

The highest sustained tropical cyclone winds on record were recorded during Typhoon Haiyan at 195 miles per hour (314 km/h) shortly before making landfall in the central Philippines on Nov. 8, 2013.[32] The most intense storm based on minimum pressure was Typhoon Tip in the northwestern Pacific Ocean in 1979, which reached a minimum pressure of 870 hectopascals (26 inHg) and maximum sustained wind speeds of 165 knots (85 m/s, 190 mph, 310 km/h).[33] The deadliest typhoon of the 20th century was Typhoon Nina, which killed nearly 100,000 in China in 1975 due to a flood that caused 12 reservoirs to fail.[34] After Typhoon Morakot landed in Taiwan at midnight on August 8, 2009, almost the entire southern region of Taiwan (Chiayi County/Chiayi City, Tainan County/Tainan City (now merged as Tainan), Kaohsiung County/Kaohsiung City (now merged as Kaohsiung), and Pingtung County) and parts of Taitung County and Nantou County were flooded by record-breaking heavy rain. The rainfall in Pingtung County reached 2,327 millimeters (91.6 in),[35] breaking all rainfall records of any single place in Taiwan induced by a single typhoon,[36] and making the cyclone the wettest known typhoon.